Theory of Light Scattering from an Object
Polarimetry is the analysis of the polarization state of light. When polarized light (the "incident light") is directed onto an object, the depolarization upon reflection for the object can be directly measured. The depolarization which occurs upon reflection can be measured by splitting the reflected light into two orthogonally oriented states and calculating the signal ratios from two light sensitive detectors that view the orthogonally oriented light beams. The detectors, with separate polarization analyzers oriented perpendicular (P.sup.1) and parallel (p.sup.2) to the direction of polarization of the incident light, measure the reflected light. The degree of depolarization is given by: EQU Depolarization(p)=P.sup.1 /P.sup.2 (1)
This ratio always yields a positive value between 0 and 1. This equation is different from that used in passive (solar-based) measurements because of the partial polarization of sunlight.
Depolarization arises due to multiple scattering of light bouncing off an object, and its magnitude is a function of the surface texture and physiology (for plants). If the surface is smooth and reflection is very specular, no depolarization will occur. This is typical of many man-made objects. On the other hand, a lambertian surface will diffuse and depolarize the reflected light when the surface is even slightly rough. Cellular materials such as plants will depolarize backscattered radiation to a degree that depends on the relative amounts of specular reflection from the surface and the diffuse reflection from the interior of the material. For a light source with a divergence .theta. and energy E located at a distance h from a target surface, the specular component, E.sub.s, of the backscatter is: EQU E.sub.s =(p.sub.s /16)(D.sub.c /htan(.theta./2)).sup.2 E (2)
where p.sub.s is the specular reflectance and D.sub.c is the diameter of the collection optics. The lambertian backscatter energy, E.sub.1, contribution to the total return signal is: EQU E.sub.1 =(p.sub.1 /4)(D.sub.c /h).sup.2 E (3)
where p.sub.1 is the fraction of the incident light that is diffusely scattered. p.sub.1 can be assumed to be 20% for a general case. Egan and Hilgeman have measured the retroreflectance from numerous materials and have reported the backscatter enhancement, R, calculated as p.sub.s /p.sub.1, to range from 0.25 to &gt;10 depending on the materials and the coherence of light.
Taking the ratio of Eqs. 2 and 3, the ratio of specular to diffuse retroreflectance is: EQU E.sub.ratio =p.sub.s /(4p.sub.1 tan.sup.2 (.theta./2)) (4)
Equation 4 shows that only the source divergence governs the backscatter enhancement of the signal. Equations 2 and 3 also provide a direct method to estimate the absolute energy of the return signal in the design of detector electronics and in the selection of light sources with sufficient power.
Wavelength Dependence of Light Scattering
The hyperspectral concept of the instant invention, particularly as it applies to the analysis of vegetation, takes advantage of the change in the reflection characteristics of leaves that occurs in the vicinity of 700 nm wavelength. Below about 700 nm, light is strongly absorbed by plant leaves and the depolarization of any scattered radiation is relatively small. Leaves usually reflect weakly in the blue and red wavelengths owing to absorption by pigments, and strongly in the infrared due to cellular refraction. At wavelengths longer than about 700 nm, the degree of depolarization is great due to multiple scattering effects which strongly depolarize the reflected light. Further, the degree of depolarization is species dependent and exhibits a characteristic wavelength dependence for each species because the leaf morphology and structure are characteristic to a particular species of plants.
Fluorescence and Vegetation Diagnostics
Radiation induced fluorescence signatures may be used to distinguish between stressed and healthy vegetation. In the instant invention, fluorescence effects can be uniquely studied by delivering only shorter wavelength radiation produced by a flashlamp to the target vegetation (by inserting a short pass filter in the output optical train) and measuring the fluorescent response from the vegetation at wavelengths longer that the wavelength of the delivered radiation.
Since fluorescent returns are small compared with the backscatter intensity, typically about 1% of the backscatter intensity, a higher collecting efficiency is required when operating in the fluorescence detection mode.
Plant stress can be directly related to the fluorescence signature. This has been demonstrated in the literature where the fluorescence signature of a stressed plant is compared with that from a healthy plant. It has been found that a healthy plant usually exhibits a peak in the fluorescence spectrum at 685 nm. On the other hand, a stressed plant shows a lowering of this peak, but a weaker peak at 735 nm increases in strength. These wavelengths are only nominal values; it is therefore useful to measure the fluorescence spectrum continuously. The ratio of the fluorescence at 685 nm to 735 nm is directly proportional to the plant health; the higher this value, the healthier the plant.
In the present invention, it is possible to measure radiation-induced fluorescence by allowing only the radiation from a flashlamp transmitted through a short-pass filter to strike the target. The instrument then measures the fluorescent response from the vegetation instead of the backscatter that would occur if the short-pass filter were not used.